I use a Wacom Grahire 3 A6 tablet that i got direct from Wacom of Germany, used, was only £45 and was in good nick.
Takes a while to get used to it, but now couldn't live without for some Photoshop stuff.
If i had the cash i'd get a bigger version as a little cramped.
Before i got the wacom i bought a Trust 1200, which didnt have any drivers, my G5 recognised it as a tablet, but i couldnt set up the stroke pressure etc, which is pretty important, so get a product that has OsX drivers.